The invention concerns an all plastic pump particularly suitable for dispensing liquid or thick substances from containers.
As it is known, dispensing pumps are widely used for dosing and dispensing liquid or creamy substances, such as soaps, cosmetic creams, detergents and similar. Said pumps are applied to the container which holds said substances.
The majority of the pumps used at present are made of both plastic materials and metal, the latter being mostly used for the return spring of the piston which forms the pump.
The presence of such metal materials entails some problems for recycling. In fact, should the pump be completely made of plastic materials, even differing from each other but being mutually compatible, it is possible to entirely recover the material the pump is made of, by shredding the pump directly. On the other hand, the presence of materials which are heterogeneous in relation to each other and, in particular, the presence of metal materials, prevents such a direct shredding and requires, for an eventual recycling of the plastic material, a previous separation of the metal parts. The difficulty and the costs related to such a sorting operation imply that the dispensing pumps, once they are no longer useful, are eliminated as a waste, with the inconvenience of wasting plastic material which could be recovered and of increasing the amount of non-degradable materials present among the waste.
Pumps are also known which, rather than using a spring made of metal material, use a bellows, as an elastic element made of plastic material obtained by a blow moulding process.
Said bellows element consists of a plurality of essentially toroidal elements, placed one on the other, so that the profile of the bellows, in a cross-section, presents itself as a series of expansions and contractions arranged on horizontal planes and essentially parallel in relation to each other.
Said type of profile can only be realized by blow moulding, because it is impossible to remove the snapping tool from the bellows after the moulding, and, therefore, its realization with the injection moulding process is not possible. The possibility for the bellows to be realized by injection moulding process rather than by blow moulding, would yield a number of advantages, one of which is the reduction of the production costs, since injection moulding is quicker than blow moulding.
Apart for this fact, the moulding of the bellows by injection would permit to obtain a thicker profile in the bending points which undergo a greater stress and, therefore, it would permit to realize a bellows having a higher mechanical resistance.
The realization of a bellows made of plastic material is known from the Belgian patent 718118. Said patent describes a bellows used in the field of distribution valves for the protection of the sliding elements of the valves themselves. The bellows realized according to the dictates of the mentioned patent presents its lateral surface in the shape of a spiral, so as to obtain the bellows by injection moulding since the spiral profile permits the removal of the snapping tool.
The bellows of this type present one profile of the spiral in their longitudinal section, which is not essentially symmetrical in relation to the plane of inclination of the spiral itself, so that they have a limited resistance when they undergo a pressing force because of the action of an axial stress which turns into a smaller build-up of elastic power and, therefore, into a weaker elastic return when they are released.
It easy to understand that such a fact consitutes a negative factor for the use of said bellows in the range of the pumps for dosing elements, wherein it is necessary to have forces presenting a rather consistent elastic return, since the piston of the pump, once it has gone back to its resting position after the dispensing action has been completed, must suck back the liquid in order to prearrange the pump for the next dispensing operation.